Conventional continuously variable transmission (CVT) systems are well known. CVT applications rely on shaft-to-shaft power transmission in their operation. The problem with CVT systems is that, at high-speed modes of operation, the pulley-belt action limits the capacity of the systems, as the belt tends to jitter due to slippage. Furthermore, CVT losses are greatest at low speeds and at overdrive, and reduced to a minimum when the velocity ratio i.sub.v is about 1.00.
To solve this problem which is quite important in automotive applications, it has already been suggested to use a continuously variable power split transmission (CVPST) system such as in U.S. Pat. No. 5,167,591 by COWAN. Such a system is actually a variable pulley system (or “variator”) coupling two of the three rotating elements of a planetary gear train. A main feature of such a system is that it carries only a fraction of the total power flowing through the variator, thus increasing the power envelope of the potential engine application. This feature also reduces the power losses associated with power transmission, especially at the low-speed-high-torque modes, while providing continuously variable transmission ratio capability. Thus, as it can be understood, CVPST systems accomplish two important functions, which are desirable in any automotive application.
First of all, at low speeds, only a fraction of the power flows through the variator thereby increasing the power envelope of the application while reducing the losses by the same factor.
Secondly, the system provides a “stepless” transmission ratio variation. This, combined with appropriate engine throttle control (motorized throttle), allows for optimum engine performance and minimum fuel consumption and emissions.